Profiled strips extruded of polymeric materials are widely used in automobile body applications as molding pieces. Molding pieces such as body side moldings or wheel well moldings are used to protect a vehicle body from impact damage and to protect edges of sheet metal panels from corrosion. Most of these molding pieces are extruded of thermoplastic materials such as PVC (polyvinyl chloride), thermoplastic rubber, thermoplastic urethane, or other flexible polymeric materials. PVC is one of the most widely used materials for its low cost and good physical properties.
Molding pieces such as a body side molding may be extruded of a plastic material alone or extruded in an extrusion coating process where plastic material is extruded on a metal reinforcing substrate. In an extrusion coating process where plastic material is combined with a more rigid metal substrate, a cross-head extrusion technique is used. In cross-head extrusion, a continuous length of metal reinforcing substrate is pulled through an extrusion die at 90.degree. angle to the direction of the plastic flow while molten plastic material is being extruded onto the metal substrate. The metal reinforcing substrate is usually cleaned and dried through a multi-station process and formed by a series of metal roll formers prior to the entry into the extrusion die.
In automobile body parts, the aesthetic appearance of the part is one of the most important criteria that must be satisfied. In the extrusion of vinyl or vinyl/metal substrate reinforced molding pieces, the quality of the surface appearance of the molding pieces obtained is dependent upon many variables. For instance, these variables include the material composition, the extrusion temperature, the extrusion pressure, the extrusion rate, the cooling rate, etc. When a high gloss surface appearance is desired on a PVC molding piece, two common approaches are normally used. First, a high gloss PVC molding composition may be used to produce molding pieces having high gloss surface appearance. This can be achieved only at a substantial cost penalty since high gloss PVC compositioning normally cost substantially more than low gloss compositions. The other approach is to permanently laminate a high gloss film to the outer periphery of a molding strip. This approach also has its drawbacks. For instance, the cost of high gloss films is often prohibitive for the automobile designer to obtain high gloss appearance on a molding piece. Moreover, laminated molding pieces always have potential delamination problems, especially when the molding pieces are subjected to severe environmental exposures.
Minor irregularities in the extrusion conditions and in the quality of the plastic resin also cause surface imperfections in the molding pieces produced. These surface imperfections include the commonly known "pimples", "blow holes", "crows feet", "shock lines" and the "salt and pepper" appearance.
It is, therefore, an object of the present invention to provide a method of extrusion casting profiled strips of polymeric material having a desirable surface finish.
It is another object of the present invention to produce extruded profiled strips of polymeric material that have substantially no surface defects and imperfections.
It is a further object of the present invention to extrude profiled strips of polymeric material having high gloss surface appearance without using permanently laminated high gloss films.